Ramanujan's approximation to population of x^2 + y^2 <= 2^n.

Terms

a(0) =1a(1) =2a(2) =3a(3) =5a(4) =9a(5) =16a(6) =29a(7) =52a(8) =94a(9) =175a(10) =327a(11) =616a(12) =1169a(13) =2231a(14) =4273a(15) =8215a(16) =15842a(17) =30628a(18) =59345a(19) =115208a(20) =224040a(21) =436343a(22) =850981a(23) =1661663a(24) =3248231a(25) =6356076a(26) =12448925a(27) =24402959a(28) =47873156a(29) =93984236